Resolve clash of object ids in SVG pgf-tikz#876

Hitherto the markers were given simply ids à la `pgf1' but this clashes
with other objects which are also numbered consecutively.  To this end
I'm simply adding a suffix `m' to the id to make is distinct from other
objects.

tex/generic/pgf/systemlayer/pgfsys-common-svg.def

@@ -891,7 +891,7 @@
       \let\pgf@sys@svg@marker@id@start\pgfutil@empty%
     \else%
       \global\advance\pgf@sys@svg@objectcount by1\relax%
-      \edef\pgf@sys@svg@marker@id{pgf\the\pgf@sys@svg@objectcount}%
+      \edef\pgf@sys@svg@marker@id{pgf\the\pgf@sys@svg@objectcount m}%
       \edef\pgf@sys@svg@add@code{%
         \noexpand\pgf@sys@svg@ref@defs{pgfs\pgf@svg@anim@marker@start}%
         \noexpand\pgfsysprotocol@literal{<marker id="\pgf@sys@svg@marker@id" markerUnits="userSpaceOnUse" orient="auto" overflow="visible">
@@ -904,7 +904,7 @@
       \let\pgf@sys@svg@marker@id@end\pgfutil@empty%
     \else%
       \global\advance\pgf@sys@svg@objectcount by1\relax%
-      \edef\pgf@sys@svg@marker@id{pgf\the\pgf@sys@svg@objectcount}%
+      \edef\pgf@sys@svg@marker@id{pgf\the\pgf@sys@svg@objectcount m}%
       \edef\pgf@sys@svg@add@code@{%
         \noexpand\pgf@sys@svg@ref@defs{pgfs\pgf@svg@anim@marker@end}%
         \noexpand\pgfsysprotocol@literal{<marker id="\pgf@sys@svg@marker@id" markerUnits="userSpaceOnUse" orient="auto" overflow="visible">